Not to be "that guy" but...
To clarify some confusion in this thread, it might be helpful to distinguish "open source" (the application) from "free" (this hosted instance of the application). Munging the two together might lead to some incorrect conclusions. Running a "free" application for others is going to have certain costs. The cost of running an "open source" application is going to depend entirely on the resources that application consumes, which, if run privately, might be a lot less.